Digital subscriber line (DSL) technologies can provide a large bandwidth for digital communications over existing subscriber lines. Examples of DSL systems include those defined by standards including asymmetric DSL 2 (ADSL2), very-high-speed DSL (VDSL), very-high-speed DSL 2 (VDSL2), G.vector, and G. fast, which is a future standard to be issued by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) Study Group 15 (SG15). These broadband access communication technologies may provide data for triple-play services, including television, internet, voice over internet protocol (VoIP) phone. When transmitting data over the subscriber lines, crosstalk interference can occur between the transmitted signals over adjacent twisted-pair subscriber lines, for example in a same or nearby bundle of lines. Crosstalk, including near-end crosstalk (NEXT) and far-end crosstalk (FEXT), may limit the performance of various DSL systems. For example, although channel capacity in the physical media dependent (PMD) layer of a DSL system may be high (e.g., near gigabits in G.fast) with a single subscriber line, when multiple subscriber lines are deployed in a same binder, actual data rate may be lower than the channel capacity due to NEXT and/or FEXT.
NEXT may be reduced or canceled via the use of synchronous time division duplexing (STDD). In the STDD mode, all subscriber lines connected to, for example, a transceiver (transmitter and receiver) located in a digital subscriber line access multiplexer (DSLAM) may be configured to either transmit downstream signals or receive upstream signals at any given time, but not simultaneously. A transceiver located in a customer premise equipment (CPE) may be configured similarly. Therefore, for the transceiver either in a DSLAM or CPE, it may either be in a transmitting mode or receiving mode. Downstream and upstream time division may allow a transceiver to avoid its own transmitter echo, and STDD may help prevent NEXT between subscriber lines.
On the other hand, FEXT may be reduced or canceled by joint processing of signals in multiple subscriber lines. Depending on whether the signals are in a downstream or upstream direction, a crosstalk precoder or canceller may be used on an operator's end of a DSL system, such as a DSLAM. For example, crosstalk precoding is a technique in which downstream signals are pre-distorted prior to transmission through a binder. A precoding matrix comprising precoder coefficients may be used to pre-distort the signals, and thus cancel FEXT that occurs between subscriber lines in the binder. The signals may then arrive at receivers located at different customer sites with reduced FEXT, thereby achieving higher data-rates.
In today's VDSL2/G.vector products, it may have been assumed that FEXT is relatively small compared to an intended user data signal. Consequently, a linear precoder and canceller may be able to cancel most or all of the FEXT. However, in customer lab tests and field trials, it has been shown that this assumption is not always true. For example, as the number of subscriber lines in a binder increases, the levels of FEXT among the subscriber lines may increase. Sometimes, the FEXT levels may become stronger than an intended signal component. In this case, the FEXT precoder/canceller coefficients may have large magnitudes, which may affect a signal component at the receiver, potentially reducing a signal-to-noise ratio (SNR). For another example, FEXT may be relatively stronger in some high frequency subcarriers. Thus, the FEXT cancellation issue may be worse in DSL systems, e.g., G.fast, which increases the high frequency band edge from, for example, 17/30 megahertz (MHz) used in VDSL2 to 100 MHz or higher. Consequently, there may be a need for improved FEXT compensation in DSL systems.